Gasoline dispensing facilities (i.e. gasoline stations) often suffer from a loss of fuel to the atmosphere due to inadequate vapor collection during fuel dispensing activities, excess liquid fuel evaporation in the fuel and vapor containment system storage tank (hereinafter referred to as “storage tank”), and inadequate reclamation of the vapors during tanker truck deliveries. Lost vapor is an air pollution problem that is monitored and regulated by both federal and state governments. Attempts to minimize losses to the atmosphere have been effected by various vapor recovery methods. Such methods include: “Stage-I vapor recovery” where vapors are returned from the vapor containment system to the delivery truck; “Stage-II vapor recovery” where vapors are returned from the refueled vehicle tank to the vapor containment system; vapor processing where the fuel/air vapor mix from the vapor containment system is received and the vapor is liquefied and returned as liquid fuel to the vapor containment system; burning excess vapor off and venting the less polluting combustion products to the atmosphere; and other fuel/air mix separation methods.
A “balance” Stage-II vapor recovery system may make use of a dispensing nozzle bellows seal to the vehicle tank filler pipe opening. This seal provides an enclosed space between the vehicle tank and the vapor recovery system. During fuel dispensing, the liquid fuel entering the vehicle tank creates a positive pressure which pushes out the ullage space vapors through the bellows sealed area into the nozzle vapor return port, through the dispensing nozzle and hose paths, and on into the storage tank.
It has been found that even with these measures, substantial amounts of hydrocarbon vapors are lost to the atmosphere, often due to poor equipment reliability and inadequate maintenance. This is especially true with Stage-II systems. One way to reduce this problem is to provide a vapor recovery system monitoring data acquisition and analysis system to provide notification when the system is not working as required. Such monitoring systems may be especially applicable to Stage-II systems.
When working properly, Stage-II vapor recovery results in substantially equal or designed exchanges of air or vapor (A) and liquid (L) between the storage tank and the consumer's gas tank. The notation “A” and the terms “air” and “vapor” are used loosely and interchangeably herein (and throughout) to refer to air and fuel vapor mix being returned from the refueled vehicle tank to the storage tank. Ideally, Stage-II vapor recovery produces an air-to-liquid (A/L) ratio very close to 1. In other words, returned vapor replaces an equal or substantially equal amount of liquid in the storage tank during refueling transactions. When the A/L ratio is close to 1, refueling vapors are collected, the ingress of fresh air into the vapor containment system is minimized and the accumulation of an excess of positive or negative pressure in the vapor containment system is prevented. This minimizes losses at the dispensing nozzle and fuel evaporation in the storage tank and leakage of excess vapors from the vapor containment system. Measurement of the A/L ratio thus provides an indication of proper Stage-II vapor collection operation. A low ratio means that vapor is not moving properly through the dispensing nozzle, hose, or other part of the system back to the storage tank, possibly due to an obstruction or defective component.
Recently, the California Air Resources Board (CARB) has been producing new requirements for Enhanced Vapor Recovery (EVR) equipment. These include stringent vapor recovery system monitoring and In-Station Diagnostics (ISD) requirements to continuously determine whether or not the systems are working properly. CARB has proposed that, when the A/L ratio drops below a prescribed limit for a single or some sequence of fueling transactions, an alarm be issued and the affected fueling point be disabled to allow repair to prevent further significant vapor losses. The proposed regulations also specify an elaborate and expensive monitoring system with many sensors that will be difficult to wire to a common data acquisition system.
The CARB proposal requires that A/L volume ratio sensors be installed at each dispensing hose or fuel dispensing point and pressure sensors be installed to measure the containment system vapor space pressure. The sensors would be wired to a common data acquisition system used for data logging, storage, and pass/fail analysis. It is likely that such sensors would comprise air-flow sensors (AFSs).
However, one issue that may occur in such a vapor recovery system employing AFS's is that a leak may occur in the vapor return passage or vapor return pipe where vapors are recovered and returned to the storage tank. If a leak occurs in the vapor return passage or vapor return pipe for a dispensing point, vapors are likely to escape outside of the vapor containment system to atmosphere thereby defeating the purpose of containing such vapors and returning them back to the underground storage tank. One method of detecting a possible leak in a vapor recovery system is to monitor the A/L ratio using an AFS for an active dispensing point to determine if the actual vapor being recovered is equal or substantially equal to the expected amount. However, this method does not always work.
For example, a defective air valve in the nozzle or vapor return pipe of a dispensing point may not close properly to block reverse vapor flow (i.e. out of the nozzle) when the dispensing point is idle. In such a case, the A/L ratio for the defective dispensing point will not be affected, because when the dispensing point is active, the vapor flow is normal and as expected.
Therefore, it may be desirable to include as part of a vapor recovery system employing AFS's the ability to detect leak conditions for dispensing points where determination of the A/L ratio for a dispensing point will not effectively detect such a leak.